In image viewing displays, such as a liquid-crystal display, birefringence by a liquid crystal etc. varies contrast etc. in accordance with variation of viewing angles. In order to prevent such contrast variation etc., there are proposed in liquid-crystal displays such a technique that disposes a retardation plate in a liquid-crystal cell, compensates optical characteristics based on birefringence, and thereby improves viewing angle characteristics. Although uniaxially or biaxially stretched films etc. are usually used as retardation plates for compensation, they fail to provide a viewing angle characteristic that is satisfactory for all liquid-crystal cells.
Japanese Patent Laid-Open No.2000-304924 official report discloses a continuous method for producing a retardation plate in which one or more heat-shrinkable films are adhered on one side or both sides of a long film composed of a thermoplastic resin, the long film is shrunk, in a state of gripped by a tenter, by function of shrinking force of the heat-shrinkable film in a width direction at a ratio (A) of no less than 0.7 to less than 1.0 times, and subsequently the obtained film is stretched and widened in the width direction concerned at a stretching ratio (%) that satisfies an equation or less (100−ratio (A)×100)×0.15, when a width of the film excluding the gripped section after the shrinking process is defined as 100.
Since the method gives stretching also in a thickness direction, it may provide a retardation plate having retardation also in the thickness direction. However, in the producing method, when in-plane main refractive indexes of the retardation plate obtained are defined as nx and ny, and a refractive index in a thickness direction is defined as nz, and nx>ny, Nz defined by Nz=(nx−nz)/(nx−ny) is −1.0<Nz<0.1, and stretching in the thickness direction has limitations, which cannot control the retardation in the thickness direction over a wide range. A long heat-shrinkable film is used in the method, is stretched by heat contraction in a thickness direction, and therefore, a resulting retardation plate obtained has an increased thickness as compared with the original long film. A thickness of the retardation plate obtained by the method is about 50 to 100 micrometers, and is not enough, as thinness required of a liquid-crystal display etc.
The present invention aims at providing a retardation plate that enables control of retardation over a wide range in a thickness direction, and providing a method for producing the same. This invention also aims at providing an optical film using the retardation plate, and furthermore providing an image viewing display using the retardation plate and the optical film.